1. Field of the Invention
The present invention relates generally to a piezoelectric filter device of an energy-trapped type, and more particularly, to a monolithic filter device utilizing a higher harmonic in the thickness-extensional vibration mode.
2. Description of the Prior Art
FIG. 1 is a schematic cross sectional view showing an example of a conventional piezoelectric filter device of an energy-trapped type. In a filter device 1, electrodes 3 and 4 are formed spaced apart from each other by a predetermined distance on the upper surface of a piezoelectric ceramic plate 2 which has been subjected to polarization processing to polarize it in the direction of thickness, while a common electrode 5 is formed on the lower surface of the piezoelectric ceramic plate 2 so as to overlap the electrodes 3 and 4 while being separated therefrom by the thickness of the piezoelectric ceramic plate 2. The filter device 1 operates as a filter by using either one of the electrodes 3 and 4 as an input electrode and using the other as an output electrode and further, connecting the common electrode 5 to a ground potential.
In the filter device 1, a filter having frequency selectivity is constituted by a single element utilizing two modes, i.e., the A mode and the S mode (respectively represented by broken lines in FIG. 1) of fundamental mode energy of the thickness-extensional vibration (TE) mode.
In order to trap the fundamental mode energy of the thickness extensional vibration mode in the piezoelectric ceramic plate 2 in this known device, the effective Poisson's ratio .sigma. of the material which constitutes the piezoelectric ceramic plate 2 must be not less than 1/3. When a material of the Pb (Zr, Ti) O.sub.3 system is to be used as the material then in order for its Poisson's ratio taking such a value, the composition in the vicinity of the morphotropic phase boundary (M. P. B.) thereof must be used, as shown in FIG. 2.
On the other hand, as obvious from FIG. 2 the resulting filter will have undesirable characteristics such as a relatively large electromechanical coupling factor kt, a relatively large dielectric constant .epsilon.r and a somewhat small mechanical quality factor Qm when using such a composition in the vicinity of the morphotrophic phase boundary (M. P. B.) of the ceramics of the Pb (Zr, Ti) O.sub.3 system.
Undesirably, various electrical characteristics such as the bandwidth, the selectivity, or the matching impedance of a ceramic filter are limited when the filter is restricted to a material having such characteristics. More specifically, it is very difficult to obtain a filter in which the bandwidth, the selectivity, the matching impedance and the like can be changed according to their usage.
Furthermore, the frequency of the filter device 1 shown in FIG. 1 is adjusted by grinding the piezoelectric ceramic plate 2 so as to reduce the thickness thereof to be equal to that of a standard product prior to forming the electrodes 3 to 5. When the filter is to be usable in the high frequency region at a frequency of 10 MHz or more, however, the thickness of the piezoelectric ceramic plate 2 must be reduced to 200 .mu.m or less. Accordingly, the piezoelectric ceramic plate 2 is liable to be cracked and chipped at the time of the above described grinding. In addition, it is very difficult to phsically hold the piezoelectric ceramic plate 2 while, for example, forming the electrodes. Consequently, in the filter device 1 having the above described structure, the upper limit of the center frequency is limited to approximately 10 MHz.
The prior art publications available at Japanese Patent Laid-Open Gazette Nos. 85613/1983 and 85614/1983 disclose a monolithic piezoelectric ceramic filter in which a plurality of groups of electrodes which include a plurality of electrodes overlapping each other in the direction of thickness, are formed on a piezoelectric ceramic plate. In this monolithic piezoelectric ceramic filter, only a higher mode of thickness-extensional vibration is forced to be excited. These publication disclose a piezoelectric filter of an energy-trapped type which can be used in a high frequency region and uses a material having an effective Poisson's ratio of less than 1/3. In this monolithic piezoelectric ceramic filter, however, the spurious response level is still large and distortion occurs in its waveform.